The present invention relates to a process for the production of inhibitors of ACE (Angiotensin Converting Enzyme) and, in particular, to a process for separation of diastereomeric mixtures of compounds of formula 1 and 2. 
wherein R1=H or R1 denotes a carboxyl-esterifying group, such as C1-C6 alkyl, or C7-C8 aralkyl.
The previously reported syntheses of Ramipril (1, wherein R1=H) use two approaches. The first approach utilizes the reaction of racemic amino esters 4a and 4b (R2 denotes a carboxyl-esterifying group, such as C1-C6 alkyl, C7-C8 aralkyl, preferably benzyl or tert-butyl) 
with a compound of formula 3, wherein the atoms indicated with an asterisk have the S configuration, 
using amide formation methods known in peptide chemistry (such as those described in CA 1,338,162, EP 79022, U.S. Pat. No. 5,977,380, ES 549789 and ES 2004804, for example) to prepare the mixture of compounds 5 and 6, 
wherein R2 denotes a carboxyl-esterifying group, such as C1-C6 alkyl, or C7-C8 aralkyl. This route gives a 1:1 mixture of diastereomers 5 and 6 from which the desired diastereomer 5 is separated using silica gel chromatography. Subsequent removal of the protecting group by hydrogenolysis or treatment with an acid or base yields Ramipril (compound of formula 1, R1=H). This approach is disclosed in EP 79022, for example. This procedure suffers from the disadvantage of requiring two additional synthetic steps to install the carboxyl protecting group of 4a and 4b and to remove the ester group on 5, and the disadvantage of requiring costly and hard-to-implement silica gel chromatographic purification to separate 5 and 6.
The compound of the formula 3 is well known (for example in European patent 037, 231) and is accessible in various ways. Several routes for the synthesis of the racemic mixture 4a and 4b have been disclosed in the patent literature (Patent EP 79022, Patent EP 50800, Patent ES 549251, Patent CN 1106386 for example) and in the literature (V. Teetz et al Tetrahedron Letters, 1984, 25(40), 4479-4482, for example).
The second approach utilizes enantiopure amino ester 4a (R2 denotes a carboxyl-esterifying group, such as C1-C6 alkyl, C7-C8 aralkyl, preferably benzyl or tert-butyl) as one partner in a coupling reaction with compound 3, using the methods commonly known in peptide chemistry (such as those described in CA 1,338,162, EP 79022, U.S. Pat. No. 5,977,380, ES 549789 and ES 2004804, for example) to prepare 5 wherein R2 denotes a carboxyl-esterifying group, such as C1-C6 alkyl or C7-C8 aralkyl. 
When the compound of formula 5 (wherein R2 denotes a carboxyl-esterifying group, such as C1-C6 alkyl or C7-C8 aralkyl) is prepared using a coupling of 4a with 3, the ester protecting group (R2) is removed by hydrogenolysis or treatment with an acid or base, then the resulting product (1, Ramipril where R1=H) is crystallized from a substantially pure solution. However, this general route also has its difficulties. Firstly, the efficient large-scale enantioselective synthesis of chirally pure 4a has not been reported. However, there are two reported enantioselective syntheses of compound 4a or derivatives. An enantioselective synthesis of 7a was reported by L. M. Harwood and L. C. Kitchen (Tetrahedron Letters, 1993, 34(41), 6603-6606) but the chemistry does not appear suitable to scale-up and the overall yield is low (13%) 
An enantioselective (but not diastereoselective) route, reported by H. Urbach and R. Henning, (Heterocycles, 1989, 28(2), 957-965) gave 4a (R2=Benzyl) in an overall yield of 5.5%, which appears to be too low for commercial implementation.
There are three reported methods for the resolution of bicyclic amino acids of this type. An enantioenriched sample of amino acid 7a was obtained by resolution of 7a and 7b is reported in patent ES 549251. Amino acid 7a can be converted to amino ester 4a by methods known to those skilled in the art. 
This resolution removes 7b from a mixture of 7a and 7b, giving, after removal of the chiral base by acidification of the residue, a 52% yield of material (7a) in unspecified optical purity. However, this resolution uses an expensive chiral amine (S)-1-(1-naphthyl)ethylamine.
The other reported resolution methods resolve the racemic mixture of 4a and 4b. A resolution separating 4a (R2=carboxyl esterifying group) from a racemic mixture of 4a and 4b using N-acyl derivatives of optically active R or S amino acids containing a phenyl nucleus has been disclosed in patent EP 115345. This procedure gives 4a (wherein R1=Benzyl) in a yield corresponding to 102.1% of theoretical, with an optical purity of 87%. This resolution, as disclosed, uses the toxic solvent dichloromethane and an expensive anti-solvent cyclohexane.
A resolution protocol for the obtention of 4a from a racemic mixture of 4a and 4b has been reported using S-mandelic acid (J. Martens and S. Lubben, Journal fur Prakt. Chemie 1990, 332(6), 1111-1117). This protocol returns 4a in high purity ( greater than 98%) but in lower yield (43% for the mandelic acid salt). Unfortunately, these two approaches require a sequence of steps when used in the production of Ramipril, as it is reported: protection of the amino acid as a carboxylic ester, free-basing the HCl salt, formation of the salt with the resolving agent, isolation of the diastereomeric salt, free-basing the diastereomeric salt, and formation of the HCl salt. Using this many steps to produce the material consumes expensive plant time and reduces efficiency.
According to a first aspect of the present invention there is provided a process for separating diastereomeric mixtures of (2S,3aS,6aS)-1-[(S)-2-[[(S)-1-(ethoxycarbonyl)-3-phenylpropyl]amino]propanoyl]octahydro-cyclopenta[b]pyrrole-2-carboxylic acid derivative of compound formula 1 and (2R,3aR,6aR)-1-[(S)-2-[[(S)-1-(ethoxycarbonyl)-3-phenylpropyl]amino]propanoyl]octahydrocyclopenta[b]pyrrole-2-carboxylic acid derivative of compound of formula 2, the process comprising:
(a) treating the mixture of 1 and 2 with a solvent or a mixture of solvents selected from a group consisting of C2-C4 nitrile solvents, C1-C6 alcohol solvents, C6-C8 aromatic hydrocarbon solvents, C3-C10 ether solvents, C3-C6 ketone solvents, C2-C7 ester solvents, C1 to C3 chlorinated solvents, and C5-C10 hydrocarbon solvents,
(b) adding an organic or inorganic acid, if desired, selected from a group consisting of benzoic acid, mandelic acid, maleic acid, fumaric acid, methane sulfonic acid, toluene sulfonic acid, hydrochloric acid, hydrobromic acid, sulfuric acid, and phosphoric acid,
(c) allowing the compound of formula 1 to precipitate and filtering the slurry to obtain a solid compound of formula 1, where R1=H or R1 denotes a carboxyl-esterifying group, such as C1-C6 alkyl, preferably tert-butyl, and C7-C8 aralkyl, preferably benzyl. 
The process involves the treatment of an equal or unequal amount of diastereomeric compounds 1 and 2 with a solvent or a mixture of solvents, treating the mixture of 1 and 2 in a solvent with an inorganic or organic acid, if necessary, stirring the mixture of 1 and 2 in a solvent or a mixture of solvents, allowing the desired isomer to precipitate with or without seeding, adding a solvent (or a mixture of solvents) if desired, and isolating the desired substantially pure compound 1 at a temperature of xe2x88x9250 to 50xc2x0 C. as a solid. The isolated product 1 may be treated with an acid or base if necessary, or subjected to hydrogenolysis if necessary to give Ramipril.
When R1 is benzyl, for example, the preferred acid salt is maleic acid and the salt produced is (2S,3aS,6aS)-1-[(S)-2-[[(S)-1-(ethoxycarbonyl)-3-phenylpropyl]-amino]propanoyl]octahydrocyclopenta[b]pyrrole-2-carboxylic acid benzyl ester maleic acid salt. This salt is new and is a useful intermediate for manufacturing the compound Ramipril.
According to a second aspect of the present invention there is provided a process for separation of a mixture of (2S,3aS,6aS)-1-[(S)-2-[[(S)-1-(ethoxycarbonyl)-3-phenylpropyl]amino]propanoyl]octahydrocyclopenta[b]pyrrole-2-carboxylic acid derivatives of the formula 1 and (2R,3aR,6aR)-1-[(S)-2-[[(S)-1-(ethoxycarbonyl)-3-phenylpropyl]amino]propanoyl]octahydrocyclopenta[b]pyrrole-2-carboxylic acid derivative of formula 2, wherein R1=H, the process comprising:
(a) treating the mixture of 1 and 2 with a solvent or mixture of solvents selected from a group consisting of C2-C4 nitrile solvents, C1-C6 alcohol solvents, C6-C8 aromatic hydrocarbon solvents, C3-C10 ether solvents, C3-C6 ketone solvents, C2-C7 ester solvents, C1-C3 halogenated solvents, and C5-C10 hydrocarbon solvents,
(b) adding an organic or inorganic base selected from a group consisting of sodium hydroxide, lithium hydroxide, potassium hydroxide, sodium bicarbonate, sodium carbonate, potassium carbonate, tert-butylamine, triethylamine, piperidine, aniline, n-butylamine or dicyclohexylamine, then filtering the slurry to obtain a solid salt of compound of formula 1.
Suitable solvents or solvent mixtures of the present invention include but are not limited to C2-C4 nitrile solvents such as acetonitrile, propionitrile and the like, C1-C6 alcohols such as ethanol, methanol and the like, C6-C9 aromatic hydrocarbons such as benzene, toluene, xylenes and the like, C3-C10 ethers such as dimethoxyethane, diethyl ether, tetrahydrofuran, diisopropyl ether and the like, C3-C6 ketone solvents such as methyl isobutyl ketone, methyl isopropyl ketone and the like, C2-C7 ester solvents such as ethyl acetate, ethyl propionate, isopropyl acetate and the like, C5-C10 hydrocarbons such as hexanes, heptanes, octanes, and the like; and C1 to C3 chlorinated solvents such as dichloromethane, chloroform and the like. Suitable organic acids for this process include but are not limited to C1-C8 acids and diacids such as benzoic acid, mandelic acid, maleic acid, fumaric acid, and the like. Suitable inorganic acids for this process include but are not limited to hydrochloric acid, hydrobromic acid, sulfuric acid, phosphoric acid, and the like. Suitable inorganic and organic bases for this process include but are not limited to sodium hydroxide, lithium hydroxide, potassium hydroxide, sodium bicarbonate, sodium carbonate, potassium carbonate, tert-butylamine, triethylamine, piperidine, aniline, n-butylamine or dicyclohexylamine, and the like.
The preferred organic acid, when required, is maleic acid, especially when R1 is a carboxyl esterifying group and preferably when R1 is benzyl. The preferred solvents are, for instance, ethyl acetate or a mixture containing ethyl acetate, acetonitrile, butyl acetate, and isopropyl acetate, which may be present in a mixture with, for instance, diisopropyl ether and/or ethanol and/or acetonitrile. The preferred precipitation temperature range is xe2x88x9215 to 30xc2x0 C. and the ratio of the diastereomeric mixture is preferably between 8:1 to 1:5 for compound of formula 1 and 2 respectively.
In the present invention, a (2S,3aS,6aS)-1-[(S)-2-[[(S)-1-(ethoxycarbonyl)-3-phenylpropyl]amino]propanoyl]octahydrocyclopenta[b]pyrrole-2-carboxylic acid derivative of formula 1 is separated from an equal or unequal amount of (2R,3aR, 6aR)-1 -[(S)-2-[[(S)-1-(ethoxycarbonyl)-3-phenylpropyl]amino]propanoyl]-octahydrocyclopenta[b]pyrrole-2-carboxylic acid derivative of formula 2, wherein R1=H or R1 denotes a carboxyl-esterifying group, such as (C1-C6) alkyl, (C7-C8) aralkyl by treatment with a solvent (or a mixture of solvents) at a temperature between xe2x88x9250 and 50xc2x0 C., treating the mixture with an organic or inorganic acid if necessary or an inorganic or organic base if necessary, stirring the mixture in a solvent or a mixture of solvents at xe2x88x9250 to 50xc2x0 C., allowing compound of formula 1 to precipitate at xe2x88x9250 to 50xc2x0 C. with or without seeding, adding a solvent or a mixture of solvents, if desired, to give substantially pure compound of formula 1, at a temperature between xe2x88x9250 and 50xc2x0 C., as a solid. Scheme 1 depicts this process. 
The isolated compound of formula 1, where R1 is defined as above, may be treated with an acid or base if necessary, or subjected to hydrogenolysis if necessary, to give Ramipril (1, where R1=H).
Suitable solvents or solvent mixtures for this separation include but are not limited to C2-C4 nitrile solvents such as acetonitrile, propionitrile and the like; C1-C6 alcohols such as ethanol, methanol and the like; C6-C9 aromatic hydrocarbons such as benzene, toluene, xylenes and the like; C3-C10 ethers such as dimethoxyethane, diethyl ether, tetrahydrofuran, diisopropyl ether and the like; C3-C6 ketone solvents such as methyl isobutyl ketone, methyl isopropyl ketone and the like; C2-C7 ester solvents such as ethyl acetate, ethyl propionate, isopropyl acetate and the like; C5-C10 hydrocarbons such as hexanes, heptanes, octanes, and the like; C1 to C3 chlorinated solvents such as dichloromethane, and the like. Suitable organic acids for this process include but are not limited to C1-C8 acids and diacids such as benzoic acid, mandelic acid, maleic acid, fumaric acid, and the like. Suitable inorganic acids for this process include but are not limited to hydrochloric acid, hydrobromic acid, sulfuric acid, phosphoric acid, and the like. Suitable inorganic and organic bases for this process include but are not limited to sodium hydroxide, lithium hydroxide, potassium hydroxide, sodium bicarbonate, sodium carbonate, potassium carbonate, tert-butylamine, triethylamine, piperidine, aniline, n-butylamine or dicyclohexylamine, and the like.
The preferred carboxyl esterifying group for this process is R1=benzyl. The preferred acid salt, when required, is maleic acid. The preferred solvents are, for instance, toluene, dimethoxyethane, ethyl acetate, isopropyl acetate, which may or be present in a mixture with, for instance, diisopropyl ether and/or ethanol and/or acetonitrile. The precipitation temperature can range between xe2x88x9250 and 50xc2x0 C. and the preferred temperature range is xe2x88x9215 to 30xc2x0 C.
The following non-limiting examples show the process for separating 1 from 2 (R1 defined as above), via the processes of the present invention.